This application relates to the use of dendrimers as a component of thermoplastic resin compositions, such as polycarbonates, to achieve desirable surface properties in molded articles.
Polycarbonate is widely used as an engineering thermoplastic owing to its unique combination of toughness, stiffness, high softening-temperature and processability. However, molded articles formed from polycarbonates have several limitations such as low scratch, abrasion and chemical resistance and susceptibility to UV degradation. To counter these limitations, coating have been applied to the surface of the molded article. For example, scratch resistant coatings applied to molded polycarbonate allows it to be used as optical and headlamp lenses. UV-protective coatings allow polycarbonate to be used in exterior applications. Coatings are also applied for decorative and aesthetic purposes. For example, molded polycarbonate articles can be coated to look like chrome, wood, painted metal and the like. Such coated articles can have a soft, luxurious feel, and provide reduced glare. One the other hand, coating are typically applied in a separate step, which reduces productivity and adds to the cost of the product. Moreover, most coating are applied by a solvent evaporation technique which creates solvent emissions and requires a solvent recovery system. This further adds to the cost of manufacturing.
The alternative to applying a coating to a molded article is to incorporate an additive within the bulk of the polycarbonate resin that provides the desired functional characteristics. For example, siloxane copolymers may be added to improve chemical resistance, while antistatic agents are added to minimize static charge build-up. Conductive carbon black has been utilized as an additive to facilitate electrostatic painting of the molded article. Unfortunately, while these additives are beneficial in the specific property for which they are added, they usually alter other material properties in an undesirable fashion. In particular, such additives frequently make the polycarbonate more brittle, and more susceptible to molding defects. In addition, they may be associated with a reduction in low-temperature ductility, softening temperature, modulus and increased plate-out.
U.S. Pat. No. 5,820,808 describes a method for making surface-modified articles using small amounts of an additive which is incompatible with the bulk polymer. The additive is dispersed within the bulk polymer as discrete droplets, and because of its incompatibility is driven to the surface in the injection molding apparatus. It has been reported, however, that adding incompatible additives to a polymer resin tends to result in delamination in injection molded articles. (Bucknall, C. B., Toughened Plastics, Applied Science, London (1977). Moreover, the addition of incompatible additives tends to reduce physical properties of the final molded article.
Thus, there remains room for improvement in the production of molded articles with desirable surface properties.
The present invention provides compositions comprising:
(a) a thermoplastic resin; and
(b) a dendrimer additive in an amount effective to alter the surface characteristics of the thermoplastic resin. These compositions can be injection molded to produce articles in which the dendrimer additive is concentrated at the surface of the article to alter the properties of the resin. By selection of the type of dendrimer additive, the resulting characteristics of the molded article may be controlled.